In order to comply with tighter emission regulations, the motor vehicle must be provided with an On Board Diagnostic (OBD) system, for checking the proper operation of the vehicle sub-systems that can affect the polluting emissions. Since the polluting emissions strongly depend on the quality of the fuel combustion into the engine cylinders, the regulations generally require the OBD system to detect also the malfunctions of the engine fuel injection system.
The fuel injection system of modern Diesel engines comprises at least a fuel injector per engine cylinder, and a fuel pump that draws the fuel from a tank and delivers it in pressure to a fuel rail connected with all the fuel injectors. The fuel injectors are generally governed by an engine control unit (ECU) according to a multi-injection pattern, which provides for each fuel injector to perform a plurality of injection pulses per engine cycle.
Each injection pulse is characterized by an individual quantity of fuel to be injected, and by a timing at which said individual quantity of fuel must be injected. The injection timing depends on the instant at which the ECU commands the fuel injector to open, also referred as Start Of Injection (SOI), which can be expressed in temporal term as well as in term of angular position of the engine crankshaft. The individual fuel quantity depends on the opening time of the fuel injector, namely the time between the instant at which the ECU commands the fuel injector to open (SOI) and the instant at which the ECU commands the fuel injector to close, also referred as Energizing Time (ET). If a malfunction of the fuel injection system arises, the individual fuel quantity actually injected by each injection pulse may not correspond to that expected in response of the respective energizing time.
In order to overcome this drawback, most ECU implements a compensation strategy that automatically correct the energizing time of each injection pulse, in order to actually achieve a desired individual fuel quantity. Nevertheless, a malfunction of the fuel injection system may also cause the timing of each injection pulse to drift with respect to that expected.
This injection timing fault is particularly due to damages occurred by the mechanical devices driving the fuel injector, to errors of the ECU computing, or to injection drifts caused by production spread or aging of the fuel injectors. Since the injection timing has a very strict relationship with the quality of the combustion within the engine cylinders, wrong injection timing can cause the polluting emissions to exceed the maximum levels set by the regulation.
As a consequence, this regulation generally provides for the OBD system to detect a malfunction of the fuel injection system when the system is unable to deliver fuel at the proper crank angle/timing (e.g. injection timing too advanced or too retarded) necessary to maintain a vehicle's NMHC, CO, NOx, and PM emissions at, or below, an applicable emission level. In order to fulfill this requirement, a known solution uses the energizing time corrections that are determined by the above mentioned compensation strategy, and detects the malfunction of the fuel injection system when said energizing time corrections exceed a calibrated threshold.
In greater detail, the known solution provides for commanding an injection pulse to inject a desired fuel quantity, for monitoring the energizing time actually used for injecting said desired fuel quantity, and for generating an alert signal if the difference between the actual energizing time and the expected energizing time exceeds the above mentioned threshold. As a matter of fact, this known solution is based on the assumption that, when the energizing time corrections are too great, the fuel injection system is malfunctioning to the point that also the injection timing is suspected to drift.
However, this assumption represents the major deficiency of this known solution, because actually there is not an immediate and necessary relationship between energizing time, injection timing and combustion quality.
In view of the above, it is at least one object to provide an improved method for detecting injection timing faults of a fuel injection system. Another object of the present invention is to achieve the above mentioned goal with a simple, rational and rather inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.